Paper webs or sheets, sometimes called tissue or paper tissue webs or sheets, find extensive use in modern society. These include such staple items as paper towels, facial tissues and sanitary (or toilet) tissues. These paper products can have various desirable properties, including wet and dry tensile strength.
Wet strength is a desirable attribute of many disposable paper products that come into contact with water in use, such as napkins, paper towels, household tissues, disposable hospital wear, etc. In particular, it is often desirable that such paper products have sufficient wet strength to enable their use in the moistened or wet condition. For example, moistened tissue or towel may be used for body or other cleaning. Unfortunately, an untreated cellulose fiber assemblage will typically lose 95% to 97% of its strength when saturated with water such that it cannot usually be used in the moistened or wet condition. Therefore, several approaches have been taken to impart wet strength to paper products.
Paper products develop dry strength in part due to interfiber hydrogen bonding. When the paper product is wetted, water disrupts the hydrogen bonds and, as a consequence, lowers the strength of the paper product. Historically, wet strength of paper products has been increased primarily by two approaches. One approach is to prevent water from reaching and disrupting the interfiber hydrogen bonds, for example, by coating the paper product. Another approach is to incorporate additives in the paper product which contribute toward the formation of interfiber bonds which are not broken or, for temporary wet strength, which resist being broken, by water. The second approach is commonly the technique of choice, especially for tissue products. In this latter approach, a water soluble wet strength resin may be added to the pulp, generally before the paper product is formed (wet-end addition). The resin generally contains cationic functionalities so that it can be easily retained by the cellulose fibers, which are naturally anionic.
A number of resins have been used or disclosed as being particularly useful for providing wet strength to paper products. Certain of these wet strength additives have resulted in paper products with permanent wet strength, i.e., paper which when placed in an aqueous medium retains a substantial portion of its initial wet strength over time. Exemplary resins of this type include urea-formaldehyde resins, melamine-formaldehyde resins and polyamide-epichlorohydrin resins. Such resins have limited wet strength decay.
Permanent wet strength in paper products is often an unnecessary and undesirable property. Paper products such as toilet tissues, etc., are generally disposed of after brief periods of use into septic systems and the like. Clogging of these systems can result if the paper product permanently retains its hydrolysis-resistant strength properties. Therefore, manufacturers have more recently added temporary wet strength additives to paper products for which wet strength is sufficient for the intended use, but which then decays upon soaking in water. Decay of the wet strength facilitates flow of the paper product through septic systems. Numerous approaches for providing paper products claimed as having good initial wet strength which decays significantly over time have been suggested.
For example, U.S. Pat. No. 3,556,932, Coscia et al., issued Jan. 19, 1971; U.S. Pat. No. 3,740,391, Williams et al., issued Jun. 19, 1973; U.S. Pat No. 4,605,702, Guerro et al., issued Aug. 12, 1986; and U.S. Pat. No. 3,096,228, Day et al., issued Jul. 2, 1983, describe additives that are suggested for imparting temporary wet strength the paper. In addition, modified starch temporary wet strength agents are marketed by the National Starch and Chemical Corporation (Bloomfield, N.J.). This type of wet strength agent can be made by reacting dimethoxyethyl-N-methylchloracetamide with cationic starch polymers. Modified starch wet strength agents are also described in U.S. Pat. No. 4,675,394, Solarek, et al., issued Jun. 23, 1987. Additional wet strength resins are disclosed in U.S. Pat. No. 3,410,828, Kekish, issued Nov. 12, 1968 and its parent, U.S. Pat. No. 3,317,370, Kekish, issued May 2, 1967.
Still other additives have been used in the papermaking process to impart a level of dry and/or wet strength to the paper product. One type of strength additive are the galactomannan gums such as guar gum and locust bean gum. These gums and their use in paper are described in more detail in Handbook of Pulp and Paper Technology, 2nd Ed., Britt, pp. 650-654 (Van Nostrand Reinhold Co. 1964), incorporated herein by reference. The galactomannan gums generally impart dry strength to paper products. Unfortuately, in addition to having dry strength, the paper products incorporating such gums tend to be harsh to the hand. Therefore, the galactomannan gums have found utility in printing and writing paper but generally have not been useful in paper products where softness is a desirable characteristic, such as toilet tissue and facial tissue.
It is also well known to those knowledgeable in pulp bleaching, that oxidative bleaching of cellulose fibers outside the optimum pH (10 or greater) and temperature conditions can result in formation of carbonyl groups in the fibers, in the form of ketones and/or aldehydes. For example hypochlorite bleaching in the neutral pH range, will produce such a result (Cellulose Chemistry and Its Applications, T. P. Nevell & S. H. Zeronian, Eds, pp. 258-260, Ellis Harwood Ltd. Pub., West Sussex, England, 1985). Chlorine bleaching without free radical scavengers, e.g. chlorine dioxide, will also produce fibers with an elevated carbonyl content (The Bleaching of Pulp 3rd Ed., R. P. Singh Ed., pp. 40-42 & 64-65, TAPPI Press, Atlanta, Ga., 1979) as will ozone bleaching to the point of fiber degradation (M. P. Godsay & E. M. Pierce, AIChe Symposium Series, No. 246, Vol. 81, pp. 9-19). However, ketone groups do not provide significant wet strength properties to the paper product. Rather, the dry and the minimal wet strength of the untreated products are determined primarily by the existence of interfiber hydrogen bonds. Intermediate oxidations that may result in the formation of aldehydes have heretofore not been desired, since the presence of aldehyde groups tends to cause yellowing of the cellulosic fibers over time.
It is also known that certain chemicals can by intent produce cellulose fibers with an elevated aldehyde content. Examples of these are sodium periodate, periodic acid and sodium or potassium dichromate at mildly acidic pH (Cellulose Chemistry and Its Applications, T. P. Nevell & S. H. Zeronian, Eds., pp. 249-253 & 260-261, Ellis Harwood Ltd. Pub., West Sussex, England 1985).
While some of the problems of providing paper products having wet strength have at least been partially ameliorated by the art, none has solved the problems in the manner or to the extent of the present invention. It is therefore an object of this invention to provide paper products, and particularly paper tissue products, that have an initial wet strength that is significantly higher than that of the corresponding paper product formed from unmodified and untreated cellulosic fibers, and which retains sufficient strength during the period of intended use. Yet another object of the present invention is to provide paper products having an initial wet strength sufficient for use of the paper product for body cleaning in the moistened condition. It is a further object of the present invention to provide tissue paper products having an initial total wet tensile strength of at least about 80 g/inch, preferably at least about 120 g/inch.
Another object of this invention is to provide paper products that have such initial wet strengths, and which also have a rate of wet strength decay sufficient for a flushable product. It is a further object of the present invention to provide paper products having such initial total wet tensile strengths and a 30 minute total wet tensile strength of not more than 40 g/inch.